1. Field of the Invention
The present invention is related to touch-screen systems and more particularly to a sync signal generator for a capacitive sensor.
2. Description of the Related Art
One-layer sensor panels are cost effective because of their simple structure. These sensor panels can be laminated directly on top of an LCD module, for example. A typical one-layer sensor panel laminated directly on top of an LCD module is shown in FIG. 1C. To realize a one layer sensor, a diamond shaped ITO layer 108 is used. The diamond ITO layer is laid out in one layer as shown. At the intersection at the center of FIG. 1C, the transmit Tx diamonds 108 are connected by a small ITO track, while the receive Rx diamonds 108 are connected with a very thin metal bridge 112, which is separated by an insulator 110 from the sensor Y-lines. The metal bridge 112 is in the same layer of the metal that is used to route the Tx and Rx lines to a controller at the side of the panel.
In the laminated capacitive sensor panel shown in FIG. 1C, the one-layer panel does not have a GND layer to shield the sensor from LCD noise. In particular, the VCOM voltage in the LCD module can generate noise that is disruptive to touch sensing. The VCOM voltage is a periodic signal used in LCD modules as is known in the art. The VCOM signal is found in a plane that usually covers the whole LCD panel and it is located nearest to the laminated sensor panel.
Referring now to FIG. 1A, the problem of VCOM noise coupling is illustrated. A portion 100 of the sensor panel according to the prior art is shown in which the VCOM layer 104, capacitive layer 106, and a representative receive line 102 are shown in plan view and in a cross-sectional view. The receive line 102 and the VCOM layer 104 in a laminated capacitive sensor panel form a capacitance CX, which is significantly large compared to the sensor cross-capacitance desired to be measured. When the VCOM voltage signal (similar to a square wave) changes, a large amount of charge is injected into this capacitance and the charge is undesirably detected at the sensor as noise. After the VCOM signal switches, it settles to a voltage level for a certain period of time, and this time period is a clean and safe time to measure the cross-capacitance of the sensor correctly.
Further cross-sectional details are shown in FIG. 1B, wherein the VCOM layer is shown to be part of an LCD layer including the VCOM layer 104A, a liquid crystal layer 104B, and TFT (Thin Film Transistor) layer 104C as is known in the art.
Referring now to FIG. 2, an HSYNC signal tells the sensor when exactly the VCOM voltage switches, and immediately after the HSYNC signal is triggered, the sensor can start the capacitance measurements. The prior art sensor system 200 includes a sensor panel and LCD module 202 in communication with the LCD driver 206, which generates the HYSNC signal. The touch sensor integrated circuit 204, which is in communication with the sensor panel and LCD module 202, receives the HSYNC signal for proper sensing of the cross-capacitance without unnecessary VCOM-generated noise. To synchronize the sensor timing with VCOM activity, the touch sensor integrated circuit 204 can tap the HSYNC signal from the LCD driver 206. However, this requires an electrical connection as shown in FIG. 2. Unfortunately, not all LCD modules have this connection ready, hence some modification may be required.
What is desired, therefore, is a touch screen controller that is able to generate its own SYNC signal to be able to operate with all LCD displays so that there is synchronization between sensing time and VCOM activity. Such a touch screen controller would be as effective as the prior art system shown in FIG. 2 to filter out noise generated by the LCD module.